The present invention relates generally to brake control valve systems for operating a rail vehicle""s air brakes, in response to brake pipe pressure in a brake pipe, and more specifically to the service portion of a brake control valve system.
Brake control valve systems for a vehicle having air brakes generally include a brake control valve responsive to the brake pipe pressure in a brake pipe to operate the brakes of a vehicle. For a reduction of brake pipe pressure, the brakes are proportionally applied. For an increase in brake pipe pressure, the brakes may be released totally or gradually. In the AAR system, the brake control valves include a service section and an emergency section which are responsive respectively to a service rate of reduction and emergency rate of reduction to apply the brakes appropriately. Each section includes a separate diaphragm or piston responsive to the brake pipe pressure on one side and a reference pressure on the other. Not only must the brake control valve differentiate between a service and an emergency application, but it must also react in a specific amount of time to apply the brakes and propagate the signal throughout the brake system. A prior art control valve is a DB60 available from New York Air Brake Corporation. The structure is also exemplified by European Patent EP-A-328 755.
Historically, the control valves have been designed to be substantially insensitive to pneumatic noise signals in the brake pipe which could cause undesired emergency (UDE) braking and undesired release (UDR). Various mechanisms have been used, for example, chokes or restrictions interconnecting the brake pipe and the reference chamber of the diaphragms, to accommodate pneumatic noise. Some of the pneumatic noise or shock is produced by the mechanical vibration of the vehicle. One solution to accommodate mechanical vibration is discussed in U.S. Pat. No. 5,387,030.
Historically, AAR-type control valves have included a small stability choke, also known as a xe2x80x9cweeper port,xe2x80x9d providing communication between the auxiliary reservoir and the brake pipe. The weeper port was included in the ABD control valve when developed in 1963 to provide the stability which previously resulted from leakage across the metal ring seals of the previous generation of AB control valves. The new ABD had an air-tight, rubber diaphragm. The weeper port provides stability during brake application by preventing undesired brake release due to slow auxiliary reservoir leakage or minor fluctuations in brake pipe. In addition, it negated the slow brake cylinder pressure increase occurring over a period of several minutes, also know as brake cylinder xe2x80x9cpressure creep.xe2x80x9d The pressure creep results from the warming of the auxiliary reservoir and its resultant pressure increase after an essentially adiabatic pressure reduction. This chilling and subsequent pressure increase of an air reservoir resulting from rapid pressure reduction and the corresponding warming and pressure loss resulting from rapid pressure increase are known commonly as the xe2x80x9ctemperature effect.xe2x80x9d
Today, with the advent of modern precision locomotive brake controls, like the CCB (Computer Controlled Brake) manufactured by NYAB and the digital pressure displays in the cab of the locomotive, the locomotive engineer has the ability to control brake pipe pressure reductions and see the result on the display screen with 1 psi accuracy. The improvements in the locomotive brake control technology have resulted in the need for corresponding improvement in the performance of the brake control valve. When a 1 or 2 psi split reduction is made, with dwell times greater than 2-3 minutes between reductions, the weeper port allows auxiliary reservoir air to flow to the brake pipe as the auxiliary reservoir pressure increases due to the temperature effect. The control valve is thus desensitized by the amount of the auxiliary pressure increase, which can be shown to be 0.5-0.6 psi. This translates to approximately 1.25 to 1.5 psi of brake cylinder pressure development. Or stated another way, for small split reductions with long dwell times between the reductions, a control valve with a weeper port will develop 1.25 to 1.5 psi less brake cylinder pressure for each small (1-2 psi) brake pipe reduction than a control valve which does not have a weeper port.
For larger brake pipe reductions, the xe2x80x9clossxe2x80x9d of brake cylinder pressure is small compared to the resulting brake pressure achieved and is not apparent to the locomotive engineer. The performance difference is most visible on long grade braking where, after the initial reduction, several very small split reductions are made over a long period of time. Each split reduction results in a brake cylinder pressure 1.25-1.5 psi lower than anticipated, which is a significant fraction of the brake pressure expected for that brake pipe reduction.
Therefore, the invention provides a uni-directional stability port in the weeper function, which will prevent the flow from auxiliary reservoir to the brake pipe during pressure increase of auxiliary reservoir from the temperature effect. Thus, the pressure increase of the auxiliary reservoir from heating up results in re-application of the control valve to increase the brake cylinder pressure by an amount proportional to that auxiliary reservoir pressure increase. On the other hand, the invention provides communication between the brake pipe and auxiliary reservoir when the brake pipe pressure is greater than the auxiliary reservoir pressure. This might occur when auxiliary reservoir is subject to a leak, or when brake pipe pressure fluctuates as a result of any one of several well-known causes. Thus, undesired brake releases are prevented.
The service portion of a rail pneumatic brake control valve of the present invention includes a piston responsive to pressure differential between brake pipe pressure at a brake pipe port and reservoir pressure at a reservoir port and a first valve controlled by the piston to determine pressure in a brake cylinder for a service application. A stability choke and check valve are connected in series between the brake pipe port and the reservoir port. The check valve permits flow from the brake pipe port to the reservoir port and prevents flow from the reservoir port to the brake pipe port independent of the piston.
The service portion further includes a charging choke and charging valve in series between the brake pipe port and the reservoir port. The charging valve is controlled by the piston. The stability choke has a greater restriction than the charging choke. The check valve opens for a pressure differential smaller than the differential pressure on the piston which opens the charging valve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.